Why is my Shadow Catcher layer shadow-pass entirely black?
As Sanix, Marasoftware and Sebastian Kónig point out (https://twitter.com/s_koenig/status/656693360955150336), the Cycles shadow pass does not work with environment texture lighting (nor does it work with mesh lighting), hence my shadow pass was entirely black.

A number of solutions were suggested, usually to identify the direction of the environment texture sun light ("Is there anyway to match a sun lamp to the direction of the sun in an environment map?") and then to mimic this light with a sun lamp, excluding it from the main layer. This made a lot of sense - and then I happened across the following:

Cycles: Shadeless &quot;Shadow-catcher&quot; Material for Rendering onto Footage

What a brilliant, simple idea! Render the shadow catcher twice, once with shadows from your main object and once without - then subtract the clean shadowcatcher from the shadowed layer, leaving only the shadow behind! (Then use RBG curves or what have you to tweak the darkness of the shadow).

Barring any future mishaps or objections from the community, I'll be using this in from now on.

Two side notes:

1) This seems like an insanely useful material for this kind of task in future, though I haven't done all my research on it yet. Could be handy for someone doing a little compositing...

2) Had some trouble with backfaces in my Cycles shadowcaster, this solved my problem. Hope it's of use.